Industrial furnaces of this kind are disclosed in German Patent 2844843. They are used especially so as to be able to harden parts of high-speed steel and other tool steels. They are also suitable, however, for other heat treatments, such as bright-annealing. Such a furnace consists of a hollow cylindrical steel housing with an opening front door which allows access to the heating chamber. The heating chamber is made from a steel jacket which is lined with a thermal insulation. On the bottom and on the roof the heating chamber is provided with a large opening for the passage of gas. These openings are closed during the heating and holding period by insulated shutters. In the cooling process cold gas which is circulated through the heating chamber flows around the charge in the heating chamber. The velocity of circulation and the degree of the recooling of the gas is controllable only by the design of the heat exchanger and blower belonging to the furnace.
A high gas velocity is what is required in order to achieve a rapid cooling of the charge. Only with a sufficiently fast heat removal is it possible to perform a hardening, for example. To achieve a rapid cooling of the charge, there exists, therefore, the need to circulate at a high velocity the quenching gas blown into the heating chamber.
The hardening of steels calls for a cooling of the workpieces from the austenitizing temperature (900.degree. C.) to room temperature at controlled rates. According to the type of steel, a heat removal is required that can be achieved only with certain environment media. The highest cooling rates are achieved with liquids. Gases have a lower thermal conductivity. By increasing the gas pressure and the circulating power it is possible to increase the heat removal to within the range of liquids. Disadvantages of liquid quenching is uncontrolled quenching, contamination of the surface with degradation products which call for complicated cleaning, and the expensive and difficult technology that is involved if the workpieces have to be annealed in a vacuum.
Gas quenching is usually performed with nitrogen gas, which except for helium and hydrogen produces the best heat removal. When nitrogen is used it is possible to raise the pressure to as much as 10 bar. With helium a further increase to 20 bar is possible. However, when these inert gases are used contamination in the oven is increased to a multiple of the pressure. Any further increase in the cooling ram in the gas is possible only by using hydrogen as the thermal transfer medium, since hydrogen has the highest thermal conductivity of all the gases, and also, due to its low density, it can be circulated with low power. With this gas all workpieces which heretofore have been quenched in liquids could be quenched in gas. An additional important advantage is the possibility of performing this quenching controlledly, which is not possible in liquids on account of Leidenfrost's phenomenon. Despite these evident advantages, high-pressure hydrogen quenching has not been achieved heretofore since the use of hydrogen at high pressure constitutes a considerable safety hazard.
It is therefore the object of the present invention to make safe practice of the process possible by a rational combination of known units of apparatus and to provide a procedure or flow sheet according to which this hydrogen quenching can be practiced. The sequence of the individual steps is to be selected such that at no time in the process can an explosive mixture form in the apparatus, and that, if some components fail, the development of a safety hazard can reliably be prevented.